jkg9:
I've never used Arduino before...
avrdude: stk500_recv(): programmer is not responding
There is an introductory section on the main page, explaining how to set up the computer's com port etc.
jkg9:
Can I add a 9V battery to Arduino just like that?
If you mean a 9volt smoke alarm battery, then NO.
Not enough current capacity for the ~200mA an Arduino(50mA) + gas sensor(150mA) draws.
A 9volt battery bank made with AA batteries, connected to the DC socket, would just be ok.
Leo..